(1) Field of the Invention
The present invention relates to a rotary wing aircraft having two main engines together with a secondary engine that is less powerful, and the invention also relates to a method of controlling the aircraft.
The term “turboshaft engine” is used to mean a power unit contributing to the propulsion and/or lift of an aircraft. On an aircraft having a rotary wing, the term “turboshaft engine” is used to designate a power unit that drives rotation of a main gearbox “MGB” that in turn drives rotation of at least one rotor of the rotary wing.
(2) Description of Related Art
An aircraft is sometimes fitted with an auxiliary power unit (APU). The auxiliary power unit may for example be used for generating electricity, or for driving hydraulic systems. However, the auxiliary power unit does not drive a rotor main gearbox on a rotary wing aircraft.
Consequently, the auxiliary power unit of an aircraft does not constitute a “turboshaft engine” in the meaning of the invention.
This invention thus comes within the field of power plants for rotary wing aircraft, such as helicopters, for example.
The present invention relates more particularly to turboshaft engines, and the characteristics of such engines vary as a function of how they are sized or “dimensioned”.
Thus, the Applicant has observed that the specific weight of a turboshaft engine depends on the power that it can deliver. The more powerful the turboshaft engine, the lower its specific weight. It should be recalled that the specific weight of an engine developing a given level of power corresponds to the weight of the engine divided by said given power.
Likewise, the specific fuel consumption of a turboshaft engine depends on the power that the turboshaft engine can deliver. It is also observed that the greater the power of a turboshaft engine, the lower its specific fuel consumption.
Under such circumstances, in terms of fuel consumption, it would appear that installing a very powerful turboshaft engine is more profitable than installing a less powerful turboshaft engine.
Nevertheless, the specific consumption of a given turboshaft engine also varies as a function of the power it delivers. Consequently, a turboshaft engine presents specific consumption that is optimized when the turboshaft engine is developing the maximum power authorized for that turboshaft engine. Specifically, when the power it develops becomes lower, the specific consumption of the engine increases.
It can be seen from the above observations that it can be difficult to dimension a turboshaft engine.
On a rotary wing aircraft, the manufacturer determines the maximum power that a turboshaft engine must deliver in order to guarantee the required performance for the aircraft. Under such circumstances, the turboshaft engine is dimensioned to deliver that maximum power.
When it is found that the power from a single turboshaft engine is not sufficient, manufacturers naturally install a plurality of turboshaft engines on their aircraft. As a result, heavy aircraft have a plurality of turboshaft engines.
It can thus be advantageous to have multi-engined aircraft. Nevertheless, the use of such multi-engined aircraft raises the problem of safety in flight in the event of an engine failing.
In particular, three configurations are used on rotary wing aircraft.
In a first configuration, the aircraft has two identical turboshaft engines that are too powerful.
Turboshaft engines are said to be “identical” when they have identical characteristics for driving a rotary member, and in particular when they are turboshaft engines having theoretical maximum powers that are equal.
Conversely, engines are said to be “unequal” when they have distinct drive characteristics, i.e. engines that generate different maximum powers.
In the first configuration, both of the engines are overdimensioned so as to ensure safe flight in the event of the other turboshaft engine failing.
Each turboshaft engine may then operate at a “standard rating” during cruising flight. The standard rating is sometimes referred to herein as the maximum continuous power (MCP) rating and the maximum continuous power rating is associated with unlimited duration of use.
Each engine may also operate at specific ratings that are used during specific stages of flight.
Thus, manufacturers have provided a rating that is referred to for convenience as the “normal specific rating”. This normal specific rating is often referred to as the “takeoff rating” because it is used during a specific stage of flight for takeoff. In a twin-engined aircraft, the normal specific rating is also used during a specific stage of flight close to hovering.
The normal specific rating associates a maximum takeoff power maxTOP with a restricted duration of use. The maximum takeoff power maxTOP is greater than the maximum continuous power MCP.
Under such circumstances, the following contingency specific ratings are used on twin-engined aircraft when one of the turboshaft engines fails:
a first contingency specific rating associating a supercontingency power with a duration of about thirty consecutive seconds, referred to as a 30″ OEI (for one engine inoperative);
a second contingency specific rating associating a maximum contingency power with a duration of use of the order of two minutes, referred to as 2′ OEI; and
a third contingency specific rating associating an intermediate contingency power with a duration of use extending to the end of a flight after an engine has failed, for example, and referred to as OEIcont.
The powers developed while using contingency specific ratings are greater than the power developed while using the standard rating.
It is thus conventional for each turboshaft engine to be dimensioned as a function of its highest contingency power, i.e. above the 30″ OEI rating. In application of the above-mentioned principles, the specific consumption of turboshaft engines while they are using the maximum continuous power MCP is thus not optimized, since the maximum continuous power MCP is very different from the maximum power that the engine can deliver.
In a second configuration, the aircraft is fitted with two identical turboshaft engines, with the use of an “impasse time” (“temps d′impasse” in French language).
On the basis of experience, it is possible to envisage ignoring the risk of failure during certain stages of flight. Under such circumstances, the turboshaft engine may be dimensioned to deliver lower levels of contingency power than would be necessary in the first configuration. The weight of the engine is then reduced, but that has the consequence of reducing its maximum continuous power MCP.
However, this second configuration can require pilots to be trained so as to minimize the durations of stages of flight in which no provision has been made for a turboshaft engine failure.
In a third configuration, the aircraft has three identical turboshaft engines. In the event of one turboshaft engine failing, the other two remain in operation to ensure flight safety.
On a given aircraft, a three-engined power plant requires turboshaft engines that are less powerful than a twin-engine power plant.
However, the use of turboshaft engines that are less powerful compared with a twin-engined aircraft is not fully optimized. It should be recalled that in terms of fuel consumption, an arrangement with a very powerful turboshaft engine is less expensive than an arrangement with a less powerful turboshaft engine.
The dimensioning of the power plant of an aircraft is thus complex, independently of the configuration that is selected.
The technological background includes document U.S. Pat. No. 4,479,619, which proposes a power transmission system for three-engine helicopters.
That solution also proposes the alternative of declutching one of the three engines.
The Applicant's Super-Frelon helicopter also had three identical engines (without clutches).
Document U.S. Pat. No. 3,963,372 proposes a power-management and engine-control technique for three-engine helicopters.
In order to mitigate the problems associated with engines that are designed so as to be overdimensioned, proposals have already been made in the past for an aircraft with a twin-engined power plant having engines with different maximum powers. This applies to document WO 2012/059671 A2, which proposes two engines with different maximum powers.
Document US 2009/186320 describes an aircraft having three turboshaft engines that appear to be identical. The aircraft includes a system for simulating the failure of a turboshaft engine.
Likewise, document U.S. Pat. No. 3,002,710 describes an aircraft having at least three engines.
Document U.S. Pat. No. 4,177,693 describes a main gearbox MGB connected to three engines that appear to be identical.
Document EP 1 175 337 describes an additional mechanical control system for a rotorcraft.
Finally, document EP 2 148 066 is also known.